1. Field of the Invention
The present invention relates to a device for mounting an ultrasonic sensor on equipment for use in a harsh environment and particularly for mounting an ultrasonic transducer on asphalt paving equipment for ranging and control applications.
2. Description of the Related Art
Ultrasonic transducers and sensors have been utilized for a variety of applications, including ranging applications for controlling quantity of asphalt aggregate in an augered hopper and thickness control of asphalt being laid on a road bed, and other widely differing applications. Unfortunately, several drawbacks exist for use of ultrasonic transducers in the largely uncontrolled, ambient environment in which many of the transducers are used.
For example, an ultrasonic transducer used for ranging in asphalt paving applications is subjected to repeated heating and cooling cycles, due to the nature of those particular applications. As a result of such repeated heating and cooling cycles, moisture from the ambient atmosphere tends to condense and collect on the back side of the transducer. Over time, sufficient moisture and contamination resulting therefrom have collected on the back side of the transducer such that the performance of the transducer becomes erratic, usually followed by total failure thereof. This problem is particularly pronounced during periods of high humidity, such as during rains storms.
It would appear that this problem could be overcome by hermetically sealing the back side of the transducer, thus isolating it from the moisture and contaminants of the ambient atmosphere. Such an approach, however, introduces a new problem. As the temperature of the gaseous medium trapped within the hermetically sealed enclosure changes, the medium attempts to expand or contract accordingly, due to the inherent thermal expansion characteristics of the medium. As a result, the pressure on the back side of the transducer within the hermetically sealed enclosure differs from the pressure on the opposing side or emitting face of the transducer. Unfortunately, the accuracy and efficiency of the transducer is dependent upon the maintenance of minimal or zero differential pressures between the face and back side thereof. In turn, the solution to this pressure differential problem would appear to be solvable by venting the back side of the transducer to the ambient atmosphere, which goes full circle to the problems hereinbefore described.
Various approaches have been utilized in an attempt to alleviate the aforementioned problems, such as by venting through a signal cable connected to the transducer or other circuitous routing, desiccants, and the like. Unfortunately, these prior attempts have met with only limited or no success.
Another potential problem which exists during use of ultrasonic transducer arises from the temperature dependency of the velocity of propagation of the ultrasonic waves through the ambient atmosphere. This problem particularly exists for the elevated temperatures normally involved in asphalt paving applications. In addition, for asphalt paving applications, the temperature of the atmosphere through which the waves traverse is constantly changing, particularly when a breeze or gusty wind is present.
One approach for reducing the inaccuracies generated by the elevated temperatures is to impose a minor obstruction of known distance from the transducer which reflects a calibrating signal or echo. The transit time of ultrasonic waves associated with the calibrating echo can be compared with the transit time of ultrasonic waves reflected from a target surface. Assuming the temperature characteristics for each of the paths for the waves generating the calibrating signal and target signal are identical, then the temperature dependency of the velocity of propagation of the ultrasonic waves along their respective paths can conceivably be eliminated. Many such applications using this approach, however, space the minor obstruction within a somewhat protected enclosure, such as within a sleeve extending forwardly from the emitting face of the transducer. Such an arrangement generally prevents the temperature characteristics of the path of the ultrasonic waves generating the calibrating signal from matching those of the path of the ultrasonic waves generating the target signal, particularly during breezy or gusty conditions.
What is needed is a mounting device which protects an ultrasonic sensor or transducer from moisture and contaminants without causing a pressure differential thereacross and which substantially or entirely eliminates temperature dependency of the velocity of propagation of the ultrasonic waves, including uses in breezy or gusty conditions.